1. Field of the Invention
The present invention relates to a driving device for driving an object and, more particularly, to a driving device for driving a movable object incorporated in, for example, a precision machine or an imaging apparatus.
2. Description of the Related Art
A conventional example will be described below with reference to a lens driving device incorporated in a photographic apparatus, such as a video camera, which is one example of a precision machine or an imaging apparatus.
FIG. 5 is a diagrammatic exploded perspective view of a conventional zoom lens mechanism incorporated in a video camera or the like. Referring to FIG. 5, a lens barrel 1 holds a lens, and a support plate 1h and an arm 1e project horizontally in opposite directions from the external peripheral surface of the lens barrel 1. Projections 1a and 1c project from one side of the support plate 1h at the front and rear end portions thereof, and holes 1b and 1d are formed so as to extend through the respective projections 1a and 1c in a direction parallel to the lens optical axis. A guide bar 2, which is secured to a support member (not shown), is relatively slidably inserted through the holes 1b and 1d. A follower pin 1g extends vertically downwardly from the underside of the portion of the support plate 1h which is near to the projection 1a. The follower pin 1g is arranged to be relatively slidably inserted into a cam slot formed in a cam plate 4 which will be described later.
A lateral groove 1f which is U-shaped in cross section is formed in the extending end portion of the arm 1e of the lens barrel 1, and a guide bar 3 which is secured to a support member (not shown) is relatively slidably inserted through the groove 1f. Accordingly, the lens barrel 1 is supported by the guide bars 2 and 3 so that it can move in a direction parallel to the lens optical axis.
The cam plate 4, which is horizontally disposed below the lens barrel 1, has a guide slot 4a which extends in a direction perpendicular to the axes of the guide bars 2 and 3, and a cam slot 4b which extends obliquely with respect to the axis of the slot 4a. Two guide pins 25 and 26 which project from a fixed member (not shown) are relatively slidably inserted into the guide slot 4a, and the cam plate 4 is supported on a fixed member (not shown) for movement in parallel with the slot 4a.
Rack teeth 4c are formed along a rear end portion of the cam plate 4, and a gear 5 is meshed with the rack teeth 4c. The gear 5 is meshed with a small-diameter gear of a stepped gear 6, and a large-diameter gear of the stepped gear 6 is meshed with a pinion gear 7 secured to a shaft 8a of a motor 8.
A slidable contact piece 9 is secured to the underside of the cam plate 4 with an electrically insulating sheet 11 interposed therebetween, and the slidable contact piece 9 has two arms 9a and 9b. Bent portions 9d and 9e are formed at the extending ends of the respective arms 9a and 9b, and the bent portions 9d and 9e are respectively in contact with a conductor part 1a and a resistor part 10b formed on a fixed detecting plate 10.
The fixed detecting plate 10 has a construction in which the conductor part 10a and the resistor part 10b are formed on an electrically insulating board 10c. The conductor part 10a is connected to a power source by a lead wire 10f. Two lead wires 10d and 10e which lead from the resistor part 10b are connected via output terminals to input terminals of a microcomputer which will be described later. In a position detecting assembly consisting of the slidable contact piece 9 and the fixed detecting plate 10, its output varies depending on what portion of the resistor part 10b is in contact with the bent portion 9e of the arm 9b of the slidable contact piece 9. In other words, a voltage applied to the conductor part 10a through the lead wire 10f is divided according to the position of the bent portion 9e which is in contact with the resistor part 10b, and the divided voltage outputs are developed on the lead wires 10d and 10e.
The operation of the conventional zoom lens mechanism having the above-described structure will be described below.
When the motor 8 is activated by a control device (not shown), the pinion gear 7, the gear 6 and the gear 5 are made to rotate about their axes, and the cam plate 4 is made to move by the rack teeth 4c meshed with the gear 5 while it is being guided by the guide pins 5 and 6 in a direction perpendicular to the axes of the guide bars 2 and 3. During this time, the arms 9a and 9b of the slidable contact piece 9 slide on the conductor part 10a and the resistor part 10b, respectively. While the cam plate 4 is being made to move in parallel with the axis of the guide slot 4a, the follower pin 1g slides in the cam slot 4b, so that a force is applied to the lens barrel 1 via the follower pin 1g in a direction parallel to the lens optical axis. Accordingly, the lens barrel 1 is made to move in parallel with the lens optical axis while it is being guided by the guide bars 2 and 3. The position of the lens barrel 1 which is in progressive movement is electrically detected by the position detecting assembly consisting of the slidable contact piece 9 and the fixed detecting plate 10. If the detection result becomes equal to a predetermined set value, the motor 8 is stopped by the control device (not shown), so that the lens held in the lens barrel 1 is automatically positioned at a desired location.
However, the aforesaid conventional driving mechanism has a number of problems. For example, because it is necessary to use numerous parts such as a motor, gears and a cam mechanism, the conventional driving mechanism is excessively large in size and weight. Accordingly, it has been difficult to reduce the size and weight of an imaging apparatus provided with such a conventional driving mechanism and it has been impossible to greatly reduce manufacturing cost. In addition, since gears are used, the backlash of the gears has made it difficult to achieve highly accurate positioning of a lens and also to control the amount of movement of the lens with high accuracy.